Starting means for cold cathode vacuum tubes



June 28, 1960 B. G. FIRTH 2,943,217

STARTING MEANS FOR COLD CATHQDE VACUUM TUBES Filed March 27, 1959 INVENTOR .Bsmvm'o 6 BY M 36 a l/u, Mafia ATTORNEYS United States Patent 2,943,217 STARTING MEANS FoR COLD CATHODE VACUUM TUBES Bernard G. Firth, Newark, N.J., assignor to Tung-Sol Electric Inc., a corporation of Delaware Filed Mar. 27, 1959, Ser. No. 802,536

8 Claims. (Cl. 313-71) The present invention relates to cold cathode ,vacuum tubes of the type wherein the cathode comprises a metal sleeve or the like coated with a porous layer containing magnesium oxide, the coating comprising a sponge-like microscopic structure. Such type of cold cathode has the characteristic of copious self-sustained electron emission once emission has been initiated and assuming the presence of an electrode at a.higher potential for attractin'g'the'emitted electrons. .During electron emission from such typeof coating a steep potentialgradient is developd in the coating,jthe potential at the-outer surface being of the order of 100 volts positive ,with respect to the metal sleeve upon which the coatingis deposited. This high potential difference accounts for the Townsend type ava-. lancheof electrons during sustained emission as explained in US. patent to Dobi schek, 2,802,127.

1957, comprises a pointed electrode maintained at ahigh negative potential for emitting, by field emission,

high velocity electrons which impinge on the cathode coating and start electron emission therefrom. Other methods for starting electron emission from the cathode coating include irradiation from a source of-ultraviolet light, either within or without the envelope containing the cold cathode and excitation by high frequency energy from a Tesla coil.

There are various disadvantages inherent in each of the previously known starting techniques. The pointed electrode requires the use of a source of high negative po-' tential. Irradiation of th e cathode with ultraviolet light or excitation with'high frequency energy require auxiliary devices for generating'such light or energy. The incandescent filament of tungsten or the like gives off particles which poison the cathode coating. The' present invention avoidsthevarious above mentioned disadvantages of the prior art starting techniques.

Briefly, the starting means of the present invention comprises an incandescent filament within the envelope, positioned between the cathode and filament, shielding means to prevent poisoning of the cathode coating by material given off by the filament and circuit connections between the filament and cathode. Prior to the present invention it was believed that if an incandescent filament were employed as a starter, the cathode coating had to be directly exposed to radiation from the filament so as to be bombarded by electrons therefrom. It was also believed that a potential source was necessary to place the filament at a potential below that of the cathode in One such v The use of an incandescent filament within the tube positioned to bombard the coating with thermally generated electrons has also been recommended.

I The presentinventioncomprises novel and efficient ice order that the thermally generated electrons emitted by the filament would be drawn to the cathode. It .wastherefore surprising to find that starting could be expeditiously effected in a cold cathode vacuum tube when a shield was interposed between an incandescent. filament and the cathode coating and the filament and cathode:

sleeve were electrically connected together without any battery or other potential source interposed therebetween.

The shield effectively protects the coating from being:

poisoned by matter thrown off by the incandescent filament. One possible theoretical explanationof the opera-. tion of the new starter is as follows: When the filament is energized it gives off photons which are reflected from other parts of the tube and impinge upon the cathode coating. Electrons liberated from the coating under the 'impact of the photons, initiate a potential gradient in the, coating such that the surface thereof becomes sufl'iciently positive to attract the thermally generatedelectrons from the filament. Additional electrons are then given'otf by" the coating and self-sustaining emission occurs over the .entire cathode surface. The difierence in potential gens, -eratedacross'the heated filament may place atleast a, part of the filamen't'at apotential below that of the cathe ode but at the filament voltages employed, lessthan. 6.3 volts, such potential difiierence is too low to, account'forl effective startingf Irrespective, however, of whether-bf? not the theoretical explanation above given is correct,; the new starter iseifectiveto insure a. rapid evolution of. electrons from the coating, electron emission continuing? after the starter filament is deenergized.

a For a better understanding of. the invention and of.

specific embodiments thereof reference the accompanying-drawing of which:

Fig. 1 is a transverse sectional view through a cold': cathode vacuum tube amplifier provided with a starter embodying the invention;

Fig. 2 is a vertical sectional view, partly in elevationof the tube of Fig. l and showing the energizing circuit.

from the starter; and

. Fig. 3 is a fragmentary circuit diagram illustrating an;

alternative energizing circuit for the starter.

The cold cathode vacuum tube amplifier diagrammati- I cally illustrated in the drawing isof the type disclosed and claimed in the pending application of Albert M.'. Skellett and the present applicant, Serial No. 745,150 filed June 27, 1958 so far as the electrode. structure is concerned- The tube comprises an envelope 2 of glass. or the like within which is centrally disposed a cathode;

sleeve 4 of nickel or other suitable. metal and on the sleeve 4 is a porous layer 6 of magnesium oxidecom '7 position, the sleeve 4and coating 6 comprising the cathode. A first or sustaining grid 8 supported on posts 10'. surrounds the cathode and a second or control grid .12. mounted. on posts 14 surrounds the sustaining grid. 1 An anode 16 of generally rectangular shape is positioned.

about the cathode and gridp-ln one side wallof the' anodeis an opening or window 18 alignedwith the grid posts} 14and 10.. Withinthe envelope Z'and-adjacent,

the" window '18 is a filament 20 supported between can ductors 22 and 24. Leads for the conductors 22 and 24 and for the other electrodes of the tube are taken out through a press 26 for connection to the external circuit. The particular circuit in which the various electrodes are connected forms no part of the present invention. A suitable circuit for a tube such as that illustrated in the drawing is disclosed in the said copending application Serial No. 745,150. It is sufficient for the present purpose to point out that the lead 28 from the anode 16 will go through a suitable load device to a source of positive potential. The lead 30 from control grid 12 will go to a point of intermediate potential and to a signal source for application of control potentials thereon. The r a Patented Junef28 may; be had to lead 32 from the sustaining grid 8 will go to a source of potential intermediate that of the anode and that of the cathode. The leads 22 and 24, in accordance with the invention, .will be connected in series with a resistor 25 across a suitable source of energizing potential which may be either direct current or alternating current. Since alternating current is more generally available, leads 22 and 24 are specifically illustrated as being connected to the secondary of a transformer 34, the primary of which is connected to alternating current lines 36. The cathode lead 38 is grounded and connected to the secondary of transformer 34 by a lead 39. A switch 40 is provided in the alternating current lines 36.

With the above described circuit and tube construction, when switch 40 is closed a voltage of substantially 6.3 volts will be impressed across the filament circuit causing the filament to incandesce. Within a fraction of a second the blue glow characteristic of electron emission will appear on the cathode surface and the tube is in operation. The switch 40 may then be opened without reduction in the electron emission from the cathode coating.

Even more rapid starting may be effected by including a battery in theconnection between the secondary of the transformer and the cathode lead 38. Such an alternative arrangement is illustrated in Fig. 3 wherein a battery 42 is shown with its positive terminal connected to cathode lead 38 and the negative terminal connected to lead 24 through a switch 44. Preferably as indicated by the dashed'line 46 switch 44 is ganged to switch 40 in lines 36 forlconjoint operation therewith. purposes, starting is sufliciently rapid without the inclusion of such a battery.

When the filament '20 is energized from a direct current, rather than from an alternating current source, the filament lead connected .to the positive terminal of such source is preferably that conductively connected to the cathode sleeve, as by the lead 39 of .Fig. 2 or through the battery 42 and switch 44 of Big. 3.

The geometry of the tube, as above described, is such that the grid posts 14 and provide shielding means that protect the cathode coating from poisoning by the starter, there being no straight uninterrupted path between the filament and cathode coating for impurities given off by the filament.

The new starting means of the invention has now been described with reference to a four electrode cold cathode vacuum tube device. Obviously the invention is not concerned with the particular structure of the other electrodes of the device as the new starting means are applicable to any vacuum tube device employing a cold cathode having a coating with the characteristics above described. ()bviously the shield positioned between the starting filament and the cathode need not be a grid support post as shown in the drawing but could be a separate shield provided solely for shielding purposes. Various alternative structures embodying the invention and within the scope of the accompanying claims will occur to those skilled in the art.

The following is claimed:

1. Starting means for a cold cathode vacuum tube device of the type wherein the cold cathode has the char- For most acteristic of copious self-sustained electron emission once emission has been initiated and comprises a metal base having an oxide coating thereon in microscopic spongelike form, said starting means comprising a filament adapted to be rendered incandescent, a shield interposed between said filament and coating to prevent contamination of the coating and a conductive connection between the metal cathode base and said filament, said cathode, filament and shield being located within a common evacuated envelope.

2. The starting means according to claim 2 wherein said device includes an anode, and a grid supported by grid posts, said grid surrounding said cathode and said anode surrounding said grid and cathode and having a window therein aligned with a grid post, said filament being positioned outside of said anode and adjacent said window, said grid post comprising said shield.

3. The starting means according to claim 2 wherein said device includes a second grid supported on grid posts, said second grid being positioned within said anode and surrounding said first grid and cathode with a grid post aligned with the anode window and with a post of said first grid, said grid posts comprising said shield.

4. The starting means according to claim 1 including lead-in wires for said filament, a source of energy connected across said lead-in wires, said conductive connection including one of said lead-in wires.

5. The starting means according to claim 1 including lead-in wires for said filament, a source of energy connected across said lead-in wires, and a source of direct curent energy having a positive terminal connected to the metal base of the cathode and a negative terminal connected to one of said lead-in wires, said conductive connection including said last mentioned source.

Q6. The starting means according to claim 5 including switch means for conjointly opening said conductive connection and disconnecting the filament from said first mentioned source.

7. The starting means according to claim 1 including a source of alternating current, a transformer having a primary winding connected across said source and a secondary winding connected to said filament for supplying energy thereto, said conductive connection comprising a lead connecting said secondary winding to the metal base of the cathode.

8. The starting means according to claim 1 including a source of alternating current, a transformer having a primary winding connected across said source and a secondary winding connected to energize said filament, a source of direct current potential having a positive terminal connected to the metal base of the cathode and a negative terminal connected to the secondary of the transformer, said conductive connection including said last mentioned source.

References Cited in the file of this patent UNITED STATES PATENTS 2,244,318 Skellett June 3, 1941 2,736,838 Nelson Feb. 28, 1956 2,802,127 Dobischek et a1. Aug. 6, 1957 2,842,706 Dobischek July 8, 1958 

